The present invention relates to a pneumatic booster attached to a brake apparatus for a vehicle, such as an automobile.
In general, a pneumatic booster is attached to a brake apparatus for an automobile, so as to generate a large braking force. As a pneumatic booster, generally, there has been known a pneumatic booster in which a housing is divided, by means of a power piston, into a constant pressure chamber (which is maintained at negative pressure under an intake pressure of an engine) and a variable pressure chamber. A valve body is connected to the power piston, and a plunger provided inside the valve body is moved by means of an input rod, to thereby introduce atmospheric air (positive pressure) into the variable pressure chamber, thus generating a pressure differential between the constant pressure chamber and the variable pressure chamber. A thrust force generated in the power piston due to the pressure differential is applied through a reaction member to an output rod. A reaction force of the output rod applied to the reaction member is partly transmitted to the input rod.
With respect to a pneumatic booster of this type, the relationship between an input force (a pedaling force acting on a brake pedal) and an output force (a braking force) is as indicated by a solid line in FIG. 27. As shown in FIG. 27, in an initial stage of a braking operation, a “jump-in” output A is generated due to the existence of a gap between the plunger and the reaction member. Thereafter, the output force increases linearly in proportion to the input force and reaches a full-load point B.
Needless to say, in the above-mentioned conventional pneumatic booster in which a braking force linearly increases in proportion to a pedaling force, a large pedaling force is required to generate a sufficiently large braking force in the event of an emergency. Therefore, to reduce a pedaling force required to generate a large braking force in the event of an emergency, a pneumatic booster comprising a so-called brake assisting mechanism is desirably employed. A pneumatic booster comprising a brake assisting mechanism, in combination with an anti-lock brake apparatus which prevents wheel lock during braking, remarkably improves a braking ability in the event of an emergency.
A pneumatic booster comprising a brake assisting mechanism is disclosed in, for example, Unexamined Japanese Patent Application Public Disclosure No. 2000-25603. In this pneumatic booster, a plunger associated with a spring is employed so that the plunger changes its effective length under expansion and compression of the spring. In the event of an emergency where a pedaling force acting on a brake pedal exceeds a predetermined level, the spring is compressed, and the plunger is displaced by a large amount relative to a valve body. Thus, a boosting ratio rapidly increases as is indicated by a portion C in FIG. 27, and a large braking force is generated.
In view of the above, the present invention has been made. It is an object of the present invention to provide a pneumatic booster having a simple structure, which is capable of rapidly developing an output force in the event of an emergency, so that a desired braking force can be reliably generated.